Type I, or insulin-dependent, diabetes mellitus (also referred to herein as DM-I) is known to occur spontaneously in humans, rats and mice (Castaxc3x1o, L and Eisenbarth, G. (1990) Ann. Rev. Immunol. 8:647-679). There is a genetic susceptibility to DM-I associated with certain haplotypes of Class II antigens of the major histocompatability complex (MHC), i.e., HLA-DR3, -DR4 and -DQ3.2 in humans (see e.g., Platz. P. et al. (1981) Diabetologia 21:108-115; Todd, J. et al. (1987) Nature 329:599-604); RT1u in Bio-Breeding (BB) rats (see e.g., Colle, E. (1990) Clin. Immunol. and Immunopathol. 57:1-9; Parfrey, N. A. et al. (1989) Crit. Rev. Immunol. 9:45-65) and H-2g7 in non-obese diabetic (NOD) mice (see e.g., Kikutani, H. and Makino, S. in Adv. Immunol. (Dixon, F. J., ed.), pp. 285-323, New York, N.Y.: Academic Press, Inc., 1992). The pathology of DM-I consists of the progressive inflammatory infiltration of pancreatic islets (i.e., insulitis) containing immunocytes targeted specifically to insulin-secreting xcex2-cells (see e.g., Bottazzo, G. F. et al. (1985) N. Eng. J. Med. 313:353-360; Foulis, A. K. et al. (1991) J. Pathol. 165:97-103; Hanenberg, H. et al. (1991) Diabetologia 32:126-134). This pathology develops over an indeterminate period of time (months to years).
It has become clear that the development of Type I diabetes occurs as a result of a complex relationship involving genetic predisposition, environmental influences, and additional undefined co-factors. In attempting to understand the pathogenesis of DM-I, the most elusive pieces of information have been the definition of the inciting autoantigen(s), and whether cellular or humoral-mediated autoreactivity is the primary event. Serum autoantibodies directed against islet cell cytoplasm and surface antigens (i.e., ICA, ICSA), insulin (IAxe2x80x2A) and glutamic acid decarboxylase (GAD) can be found in pre-diabetic and newly diagnosed diabetic humans and animals (see e.g., Rowley, M. et al. (1992) Diabetes 41:548-551; Palmer, J. et al. (1983) Science 222:1337-9; MacLaren, N. et al. (1988) Diabetes 38:534-538; Baekkeskov, S. et al. (1990) Nature 347:151-156; Velleso, L. A. et al. (1993) Diabetologia 36:39-46). Unfortunately, correlations between antibody titers against these antigens and the clinical onset of diabetes have not been successfully predictive. In addition, a number of other xcex2-cell antigens become exposed as islets are destroyed such as a 69kd islet cell autoantigen (ICA69) (Pietropaolo, M. et al. (1993) J. Clin. Invest. 92:359-371), 38kd and 62kd insulin secretory granule proteins (Roep, B. O. et al. (1991) Lancet 337:1439-1441; Brudzynski, K. et al. (1992) J. Autoimm. 5:453-463) and proislets (Harrison, L. C. et al. (1992) J. Clin. Invest. 89:1161-65; Harrison, L. et al. in Advances in Endocrinology and Metabolism (Mazzaferri, E. L. et al., ed.), pp. 35-94, St. Louis, Mo.: Mosby-Year Book, 1990). However, it is not clear whether the cellular and/or humoral immune responses to these antigens are the cause or simply a consequence of ongoing islet cell damage. In short, the immunologic nature of the pathogenic mechanism and the exact antigen(s) inducing the diabetogenic attack have yet to be elucidated.
Over one half million people in the United States suffer from insulin-dependent diabetes. Prior to 1921, people who developed DM-I were not expected to live much more than a year after diagnosis. Afflicted individuals suffered from clinical signs of chronic hyperglycemia (e.g., excessive thirst and urination, rapid weight loss) as a consequence of abnormal carbohydrate metabolism. Once insulin was purified and administered, the life-expectancy of diabetics increased dramatically. However, DM-I is a chronic disease that requires life-long treatment to prevent acute illness and to reduce the risk of long-term complications. Restrictive diets and daily insulin injections can be burdensome for patients, thus reducing compliance, and even with treatment complications such as cataracts, retinopathy, glaucoma, renal disease and circulatory disease are prevalent.
Accordingly, more effective treatments for Type I diabetes are needed, in particular therapies that address the autoimmune basis of the disease, rather than merely treating the symptoms. Additionally, given that the xe2x80x9cpre-diabeticxe2x80x9d phase of DM-I is long in duration and clinically asymptomatic, one important opportunity for therapeutic intervention falls during this period. However, effective diagnostic assays that can identify people in this pre-diabetic phase are lacking. Methods that would enable identification of early or mounting xcex2-cell abnormalities in individuals predisposed to diabetes are needed and would allow treatment early in the disease process, which may help to avert life-long insulin dependence.
This invention pertains to proinsulin peptide compounds which modulate an immunological response by T cells of Type I diabetic subjects. In one embodiment, a proinsulin peptide compound of the invention stimulates an immunological response by the T cells. For example, humans with DM-I have greater numbers of circulating T cells which respond to a specific proinsulin peptide described herein than do non-diabetic control humans. Accordingly, a subject""s immunological responsiveness to a stimulatory proinsulin peptide compound can be used as an indicator of DM-I. In another embodiment, a proinsulin peptide compound of the invention inhibits an immunological response by T cells of Type I diabetic subjects. The invention further provides therapeutic and preventative methods involving the use of the proinsulin peptide compounds of the invention to inhibit or prevent T cell responsiveness to proinsulin in Type I diabetic subjects.
In a preferred embodiment, the proinsulin peptide compound that modulates an immunological response from T cells of Type I diabetic subjects is derived from a region of proinsulin that spans the junction between the B chain and the C peptide of proinsulin. In another embodiment, the proinsulin peptide compound is a modified form of a proinsulin peptide derived from this region. Such modified forms include peptides that have amino acid substitutions compared to the native proinsulin amino acid sequence yet retain certain structural and functional features of the native peptide. Other modified forms of the proinsulin peptide compounds within the scope of the invention include peptides with end-terminal or side chain covalent modifications and peptide analogs and mimetics. Such modified proinsulin peptides can be selected for altered properties of the peptide, e.g., stability, solubility, immunogenicity, etc. The proinsulin peptide compounds of the invention can be incorporated into pharmaceutical compositions suitable for administration to a subject.
Another aspect of the invention pertain to a method for detecting an indicator of Type I diabetes in a subject by detecting an immunological activity against a proinsulin peptide compound of the invention in a biological sample from the subjects.
Yet another aspect of the invention pertains to a method for inhibiting the development or progression of Type I diabetes in a subject by administering to the subject a proinsulin peptide compound which modulates an immunological response by T cells of Type I diabetic subjects.